a) Field of the Invention
The invention is directed to a drying device for drying a gas, particularly a gas that is guided in a circuit through a drying vessel of a drying plant, which drying vessel receives material to be dried, with a gas input line for supplying the gas to be dried and with a gas output line for discharging the dried gas, and with parallel cooling branches which can be switched by a switching device between an active state, in which the gas to be dried flows through the cooling branch and cold-drying of the gas is carried out in the cooling branch, and a passive state, wherein at least one of the cooling branches is in its active state at a given time, and wherein a gas-coolant heat exchanger arranged in each of the cooling branches has a first channel through which the gas to be dried flows in the active state of this cooling branch and a second channel which, in the active state of this cooling branch, forms an evaporator for a coolant circulating in at least one coolant circuit.
b) Description of the Related Art
Drying devices for drying a gas, in particular air, are used, for example, in connection with plants for drying bulk material such as plastic granulate. After a certain storage period or after production, bulk material of this kind can have too much moisture for processing, e.g., in extruders or injection molding machines, particularly when it is hydroscopic. The bulk material must then be dried before processing. Drying plants with closed circuits for the drying air are already known for this purpose. A drying plant of this kind is known, for example, from EP 626 191 A1. Air is guided by means of a conveyor device through an adsorption dryer and is then heated and blown into the bulk material vessel. After flowing through the bulk material, the air is fed to the input side of the conveyor device. After a certain operating time, the adsorbent medium of the adsorption dryer is saturated with moisture so that the desired dew point of the air, e.g., −40° C., is no longer reached after the air flows through the adsorbent medium. A regeneration cycle must then be carried out. For this purpose, the conveying direction of the conveyor device is reversed and ambient air is heated and guided through the adsorbent medium so that the adsorbent medium is regenerated by the hot air. This drying plant is disadvantageous due to the required servicing because the adsorbent material must constantly be changed in a time-consuming manner and the total energy consumption of the device is still relatively high.
Another drying device for drying a gas, in particular air, in the form of an adsorption dryer is disclosed in DE 32 26 502 A1 in which the adsorbent medium is additionally cooled in order to increase the moisture absorbing capacity of the adsorbent medium.
In another type of known drying plant for bulk material, dried compressed air is supplied through a compressed-air line. A pressure reduction valve reduces the pressure of the supplied compressed air to a preset value. The air is heated to the desired temperature by a heating device and subsequently flows into the bulk material vessel in the vicinity of its take-off opening. After flowing through the bulk material, the moisture-laden air flows through an exhaust outlet. In order to dry the dried compressed air that is supplied, the compressed air is usually guided through a molecular sieve dryer, after which its dew point is in the range of −40° C. to −50° C., for example. For pre-drying the compressed air, the latter can be dried initially to a dew point temperature of 3° C. by a commercially available cold dryer before flowing through the molecular sieve dryer, wherein condensation will occur. Accordingly, the compressed air supplied to the molecular sieve dryer has a dew point of 3° C. The regular servicing of the compressed-air compressor and the high energy consumption are disadvantageous in this drying plant.
A device of the type mentioned in the beginning is known from DE 198 08 011 C1. This device for cold-drying of the gas to be dried has two parallel branches through which the gas is guided in opposing cycles and cold-dried. The branch through which the gas is not currently guided is regenerated and ice deposits are thawed and removed. For this purpose, a partial flow of the hot gas supplied to the drying device is branched off from the main flow before the gas is cooled and is guided through the currently passive cooling branch. This partial flow is then fed, together with the main flow, to the other cooling branch. Among the disadvantages in this device is the fact that the additional moisture absorbed by the partial flow in the passive cooling branch leads to a faster icing of the active cooling branch so that more frequent switching processes are required between the branches. This decreases the overall efficiency of the plant.